Amitriptyline and nortriptyline are tricyclic antidepressant drugs which are prescribed for the treatment of chronic depression and are represented by Formula I and Formula II, respectively: ##STR3##
When amitriptyline is the primary drug for such treatment, nortriptyline is usually present as a naturally occurring metabolite produced by demethylation of the tertiary nitrogen of amitriptyline. Accordingly, nortriptyline is not only present when it is prescribed as the primary drug for treatment of chronic depression, but is also present when amitriptyline is employed as the primary drug for such treatment.
The monitoring of therapeutic drug levels of amitriptyline and nortriptyline in biological fluids such as serum, plasma, whole blood, urine, and the like, has become very useful to provide physicians with information to aid in patient management. The monitoring of such drug levels enables adjustment of patient dosage to achieve optimal therapeutic effects, and helps avoid either subtherapeutic or toxic levels, especially in the case of treatment with amitriptyline which results in the presence of both amitriptyline and nortriptyline. In this regard, since high levels of amitriptyline and nortriptyline have been associated with central nervous system disorders, cardiovascular toxicity, hypertension, seizures, coma and death, the concentration of amitriptyline and nortriptyline in a patient's blood must be maintained in a therapeutic range, particularly since a wide interpatient variation normally exists in human plasma for a given dose.
Accordingly, since individuals vary greatly in their response to treatment with amitriptyline or nortriptyline, it is necessary to monitor the therapy by measuring both the levels of amitriptyline and nortriptyline where amitriptyline is the primary drug for treatment, or measuring the level of nortriptyline where nortriptyline is the primary drug for treatment, in, for example, the serum or plasma of the patient. Concentrations below the desired therapeutic ranges are proposed to be subtherapeutic for the treatment of depression, while levels higher than the range can be associated with undesirable effects including cardiovascular complications, anticholinergic effects, and sedation, without any increase in antidepressant efficacy.
The measurement of amitriptyline and nortriptyline levels by chromatographic techniques, such as high pressure liquid chromato-graphy [Dorey, et al., Clin. Chem., 34, 2348-2351 (1988)], gas chromatography [Garland, et al., Clin. Pharmacol. Ther., 25, 844-856 (1979)], thin-layer chromatography [Nagy, et al., J. Pharm. Pharmacol., 25, 599-603 (1973)], have been described. However, such techniques are labor intensive, requiring highly skilled personnel to perform various cumbersome steps which are time consuming and tedious.
The immunoassay determination of the levels of tricyclic antidepressant drugs, such as by radioimmunoassay (RIA) techniques [Sayegh, Neurochem. Res., 11, 193-206 ( 1986); Virtanen, Scand. J. Clin. Lab. Invest., 40, 191-197, (1980)], by enzyme linked immunosorbent assay (ELISA) techniques [Denis, et al., Clin. Chem. Acta, 159, 257-267 (1986)], by fluorescence polarization immunoassay (FPIA) techniques [U.S. Pat. No. 4,420,568 and European Patent Application No. 226,730], and by enzyme immunoassay (EIA) techniques [U.S. Pat. No. 4,307,245, U.S. Pat. No. 4,223,013 and Pankey et. al., Clin. Chem., 32, 768-772 (1986)], have been described. However, these techniques suffer from either a lack of specificity, i.e. determination of amitriptyline in the presence of nortriptyline and amitriptyline metabolites and determination of nortriptyline in the presence of amitriptyline and nortriptyline metabolites, or require labor-intensive column purification to overcome the lack of antibody specificity in the presence of the analyte's metabolites. In particular, a non-specific fluorescence polarization immunoassay (FPIA) for the detection of the total amount of the four major tricyclic antidepressant drugs is commercially available and described in European Patent Application Publication No. 226,730 and U.S. Pat. No. 4,420,568 wherein the concentration determined by this assay is only an estimation of the total amount of tricyclic antidepressant in plasma or serum. Accordingly, such assay cannot be used to accurately quantify a specific individual tricyclic antidepressant drug, for example, in a patient treated with amitriptyline, but instead, would give the total amount of amitriptyline and nortriptyline in the patient's plasma or serum.